42 Overlapping propellers

Where two propellers are fitted, these can be made to overlap (Pien and Strom-Tejsen, 1967; Munk and Prohaska, 1968) (Fig. 4.1). As early as the 1880s, torpedo boats were fitted with overlapping propellers by M. Normand at the French shipyard. The propellers turned in the same direction partially regaining the rotational energy. Model tests in Germany in the 1970s covered only cases for oppositely turning propellers. Better results were obtained for propellers which turned outside on the topside.

Overlapping propellers have rarely been used in practice, although the theory has been extensively investigated in model tests. It differs from conventional arrangements in the following ways:

1. The total jet area is smaller—this reduces the ideal efficiency.

2. The propellers operate in an area of concentrated wake. This increases hull efficiency iqh = (1 — 0/(1 — w).

3. There may be some effects from mutual interaction.

4. Parallel shafts with a small axial separation provide less propeller support. Propeller support is improved if the smaller propeller separation is used with a rearwards converging shaft arrangement. This also makes engine arrangement easier.

5. Recovery of rotational energy with both propellers turning in the same direction.

6. The resistance of open-shaft brackets and shafts placed obliquely in the flow is lower than in the conventional twin-screw arrangement.

parallel shafts

The decrease in jet area and the possibility of utilizing the concentrated wake mutually influence efficiency. The overall propulsion efficiency attained is higher than that using a conventional arrangement. The resistance of the struts and shafts is reduced by around one-third with subsequent reductions in required power.

Overlapping propellers with aft slightly converging shafts feature two advantages:

+ Engine arrangement is easier. + The course-changing ability is increased.

The convergence of the shafts leads to a strong rudder moment if only one of the propellers is working. Therefore it should be determined in model tests whether the ship is able to steer straight ahead if one of the propulsion systems fails. Such a check is highly recommended for convergence angles (towards the centreplane) of 3° or more.

Interaction effects can cause vibration and cavitation. Both can be overcome by setting the blades appropriately. The port and starboard propellers should have a different number of blades.

The following quantities influence the design:

1. Direction of rotation of the propeller.

2. Distance between shafts.

3. Clearance in the longitudinal direction.

4. Stern shape.

5. Block coefficient.

The optimum direction of rotation with regard to efficiency is top outwards. The flow is then better at the counter and has less tendency to separate. Sometimes an arrangement with both shafts turning in the same direction may be better owing to energy recovery.

The optimum distance between the shafts is 60-80% of the propeller diameter (measured on a containership). The separation in the longitudinal direction has only a slight effect on efficiency and affects primarily the level of vibration.

The U-shaped transverse section, used in single-screw vessels, particularly favours this propeller arrangement—unlike the V form usually found on twin-screw vessels. The overlapping propeller arrangement has more advantages for fuller hull forms, since the possibilities for recovering wake energy are greater. Some of the advantages gained in using overlapping propellers can also be attained by arranging the propellers symmetrically with a small distance between the shafts. With overlapping propellers a single rudder can be arranged in the propeller stream.

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  • gustava
    What is overlapping propellers used?
    2 years ago

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